Missing some faces when importing from blender to unity even with correct normals
I have seen your question. The problem is with using normal maps. Normal Maps can have some errors. You should rather go with displacement maps, but I wouldn't recommend it because it eats up a lot of space. I would say you model each individual plank and then arrange them in your way and then join them. Or if you would like control over each individual plank in unity, then export it without joining. This may seem tedious but in the long run is more useful and gives you more control. I am guessing the normal maps are the planks itself, but if the normal maps are the texture and detail on the planks, then I would say go with displacement maps.
If it still doesn't work, then it's a problem with Unity. I would suggest you duplicate the model, rotate it on the X-axis and place it below the original planks. It would give it some depth and it will make your problem look better.